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										01. 多线程基础
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										03. synchronized
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										04. AQS和ReentrantLock
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										05. 线程池
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										07. CountDownLatch和CyclicBarrier
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										08. 为什么需要Java内存模型
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										09. 深入浅出Java内存模型
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										03-Java容器
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										01. List集合
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										02. Map集合
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										04-Java虚拟机
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										01. Java编译到执行的过程
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										02. 双亲委派机制
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										03. JVM内存结构
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										04. 垃圾回收机制
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										05. CMS垃圾收集器
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										06. G1垃圾收集器
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										07. JVM调优
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										01. SpringMVC
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										03. SpringBean生命周期
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										01. Redis基础
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										02. Redis持久化
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										03. Redis主从架构
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										04. Redis分片集群
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										07-消息队列
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										01. Kafka基础
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										02. 使用Kafka会考虑什么问题
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										08-MySQL
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										01. MySQL索引
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										02. MySQL事务和锁机制和MVCC
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										03. MySQL调优
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										09-项目场景类
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										01. 如何实现去重和幂等
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										02. 系统需求多变如何设计
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										03. 设计模式
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										10-计算机网络
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										01. HTTP
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										11-算法
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										01. 排序算法
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	02. 双亲委派机制
</h1>
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	<span>3y</span>
	<span>2021-08-19 17:26:04</span>
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	<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>要不你今天来详细讲讲双亲委派机制？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯，好的。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：上次提到了：class文件是通过「类加载器」装载至JVM中的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：为了防止内存中存在多份同样的字节码，使用了双亲委派机制（它不会自己去尝试加载类，而是把请求委托给父加载器去完成，依次向上）</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：JDK 中的本地方法类一般由根加载器（Bootstrp loader）装载，JDK 中内部实现的扩展类一般由扩展加载器（ExtClassLoader ）实现装载，而程序中的类文件则由系统加载器（AppClassLoader ）实现装载。</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1grxyqvo1sij30yw0u0n43.jpg"></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：这应该很好理解吧？</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：雀食(确实)！</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>顺着话题，我想问问，打破双亲委派机制是什么意思？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：很好理解啊，意思就是：只要我加载类的时候，不是从APPClassLoader-&gt;Ext ClassLoader-&gt;BootStrap ClassLoader 这个顺序找，那就算是打破了啊</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：因为加载class核心的方法在LoaderClass类的loadClass方法上（双亲委派机制的核心实现）</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：那只要我自定义个ClassLoader，重写loadClass方法（不依照往上开始寻找类加载器），那就算是打破双亲委派机制了。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：这么简单？</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯，就是这么简单</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>那你知道有哪个场景破坏了双亲委派机制吗？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：最明显的就Tomcat啊</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：详细说说？</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：在初学时部署项目，我们是把war包放到tomcat的webapp下，这意味着一个tomcat可以运行多个Web应用程序（：</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：是吧？</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯..</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：那假设我现在有两个Web应用程序，它们都有一个类，叫做User，并且它们的类全限定名都一样，比如都是com.yyy.User。但是他们的具体实现是不一样的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：那么Tomcat是如何保证它们是不会冲突的呢？</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：答案就是，Tomcat给每个 Web 应用创建一个类加载器实例（WebAppClassLoader），该加载器重写了loadClass方法，优先加载当前应用目录下的类，如果当前找不到了，才一层一层往上找（：</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：那这样就做到了Web应用层级的隔离</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtvgubhapzj60jo0d60tn02.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>嗯，那你还知道Tomcat还有别的类加载器吗？</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：嗯，知道的</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：并不是Web应用程序下的所有依赖都需要隔离的，比如Redis就可以Web应用程序之间共享（如果有需要的话），因为如果版本相同，没必要每个Web应用程序都独自加载一份啊。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：做法也很简单，Tomcat就在WebAppClassLoader上加了个父类加载器（SharedClassLoader），如果WebAppClassLoader自身没有加载到某个类，那就委托SharedClassLoader去加载。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：（无非就是把需要应用程序之间需要共享的类放到一个共享目录下嘛）</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯..</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：为了隔绝Web应用程序与Tomcat本身的类，又有类加载器(CatalinaClassLoader)来装载Tomcat本身的依赖</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：如果Tomcat本身的依赖和Web应用还需要共享，那么还有类加载器(CommonClassLoader)来装载进而达到共享</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：各个类加载器的加载目录可以到tomcat的catalina.properties配置文件上查看</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：我稍微画下Tomcat的类加载结构图吧，不然有点抽象</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gry3okwpunj311s0u0qci.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯，还可以，我听懂了，有点意思。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：<strong>顺便，我想问下，JDBC你不是知道吗，听说它也是破坏了双亲委派模型的，你怎么理解的。</strong></p>
<p><span style="color:#000080"><strong>候选者</strong></span>：Eumm，这个有没有破坏，见仁见智吧。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：JDBC定义了接口，具体实现类由各个厂商进行实现嘛(比如MySQL)</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：类加载有个规则：如果一个类由类加载器A加载，那么这个类的依赖类也是由「相同的类加载器」加载。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：我们用JDBC的时候，是使用DriverManager进而获取Connection，DriverManager在java.sql包下，显然是由BootStrap类加载器进行装载</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：当我们使用DriverManager.getConnection()时，得到的一定是厂商实现的类。</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：但BootStrap ClassLoader会能加载到各个厂商实现的类吗？</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：显然不可以啊，这些实现类又没在java包中，怎么可能加载得到呢</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯..</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：DriverManager的解决方案就是，在DriverManager初始化的时候，得到「线程上下文加载器」</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：去获取Connection的时候，是使用「线程上下文加载器」去加载Connection的，而这里的线程上下文加载器实际上还是App ClassLoader</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：所以在获取Connection的时候，还是先找Ext ClassLoader和BootStrap ClassLoader，只不过这俩加载器肯定是加载不到的，最终会由App ClassLoader进行加载</p>
<p><img src="https://tva1.sinaimg.cn/large/008i3skNgy1gtvgvgthyhj60qo08qmy102.jpg"></p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：嗯..</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：那这种情况，有的人觉得破坏了双亲委派机制，因为本来明明应该是由BootStrap ClassLoader进行加载的，结果你来了一手「线程上下文加载器」，改掉了「类加载器」</p>
<p><span style="color:#000080"><strong>候选者</strong></span>：有的人觉得没破坏双亲委派机制，只是改成由「线程上下文加载器」进行类加载，但还是遵守着：「依次往上找父类加载器进行加载，都找不到时才由自身加载」。认为”原则”上是没变的。</p>
<p><span style="color:#ab4642"><strong>面试官</strong></span>：那我了解了</p>
<p><strong>本文总结</strong>：</p>
<ul>
<li><p><strong>前置知识：</strong>JDK中默认类加载器有三个：AppClassLoader、Ext ClassLoader、BootStrap ClassLoader。AppClassLoader的父加载器为Ext ClassLoader、Ext ClassLoader的父加载器为BootStrap ClassLoader。这里的父子关系并不是通过继承实现的，而是组合。</p>
</li>
<li><p><strong>什么是双亲委派机制：</strong>加载器在加载过程中，先把类交由父类加载器进行加载，父类加载器没找到才由自身加载。</p>
</li>
<li><p><strong>双亲委派机制目的：</strong>为了防止内存中存在多份同样的字节码（安全）</p>
</li>
<li><p><strong>类加载规则：</strong>如果一个类由类加载器A加载，那么这个类的依赖类也是由「相同的类加载器」加载。</p>
</li>
<li><p><strong>如何打破双亲委派机制：</strong>自定义ClassLoader，重写loadClass方法（只要不依次往上交给父加载器进行加载，就算是打破双亲委派机制）</p>
</li>
<li><p><strong>打破双亲委派机制案例：</strong>Tomcat</p>
<ul>
<li>为了Web应用程序类之间隔离，为每个应用程序创建WebAppClassLoader类加载器</li>
<li>为了Web应用程序类之间共享，把ShareClassLoader作为WebAppClassLoader的父类加载器，如果WebAppClassLoader加载器找不到，则尝试用ShareClassLoader进行加载</li>
<li>为了Tomcat本身与Web应用程序类隔离，用CatalinaClassLoader类加载器进行隔离，CatalinaClassLoader加载Tomcat本身的类</li>
<li>为了Tomcat与Web应用程序类共享，用CommonClassLoader作为CatalinaClassLoader和ShareClassLoader的父类加载器</li>
<li>ShareClassLoader、CatalinaClassLoader、CommonClassLoader的目录可以在Tomcat的catalina.properties进行配置</li>
</ul>
</li>
<li><p><strong>线程上下文加载器：</strong>由于类加载的规则，很可能导致父加载器加载时依赖子加载器的类，导致无法加载成功（BootStrap ClassLoader无法加载第三方库的类），所以存在「线程上下文加载器」来进行加载。</p>
</li>
</ul>
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